The present invention related to the storage and retrieval of information from optical media, and more particularly, to a system for reducing the feedthrough between the focus and tracking servo mechanisms of an optical disk drive.
Optical disk drives are in widespread use and include video disk drives used in video applications as well as optical disk drives that serve as peripheral memory storage devices in computer systems. The matter category of drives includes write-once-read-many (WORM) drives, CD-ROM drives and magneto-optic (erasable) drives.
In one form of optical media, information is encoded in concentric or spiral tracks on the surface of a disk by physically altering minute regions of the media to change the reflectivity thereof. Data is written and read from the optical media utilizing a laser beam which is reflected off of the media. It is critical to maintain the laser beam in focus at the appropriate level of the optical media. Optical focusing systems are well known for this purpose and typically include a servo mechanism It is also important to accurately follow the pre-selected data track. Tracking systems for optical disk drives are well known and also typically include a servo mechanism.
Optical focus servo mechanism encounter problems when the light beam crosses between adjacent grooved data tracks. During track crossings, the diffracted order components in the reflected beam vary in a way that can introduce error in the focus signal such that the objective lens is moved completely out of optimal focus. Light amplitude variations caused by the groove structure are particularly troublesome during a radial seek operation. This undesirable phenomenon results in cross-talk between the focus error and track error signals.
One of the most popular focus servo systems utilizes an astigmatic lens and a quad detector. See Principles of Optical Disc Systems by G. Bouwhis et al., 1985, pp 79-80. While this system is capable of focusing on smooth, ungrooved surfaces, the presence of data tracks in the form of grooves alters the behavior of the system and leads to undesirable feedthrough or cross-talk.